Surprising formation of a rhodochrosite-like (MnCO3) phase on Co-Zn-Mn sintered spinels upon storage at room temperature and ambient air

Abstract

A detailed XPS investigation, supported by XRD analysis, was performed on Co-Zn-Mn oxide-based spinels as a function of cobalt concentration and calcination temperature (723 and 973K). Results revealed for the first time that a recarbonation occurred under ambient conditions but only for the Co-containing materials calcined at 973K. Indeed, such effect was not observed for the homologous preparations at 723K. This evidence was quite surprising because the solids calcined at 973K, for which a sintering occurred as reflected by the low surface areas, were supposed to be almost inert under ambient conditions. Such an intriguing recarbonation chemistry, leading even to a bulky rhodochrosite-like (MnCO3) phase in the cobalt most enriched samples, arises from the combined effect of the calcination treatment at high temperature and the presence of cobalt ions. The spinels calcined at higher temperature are characterized by a high surface defectivity, due to anionic vacancies produced by a partial self-reduction of a fraction of Mn3+ ions that occurs only at 973K but not at 723K. Consequently, the presence of anionic vacancies and the incomplete coordination of the outermost layers promoted a higher reactivity of the solids calcined at 973K with CO2 and moisture from ambient air. However, a carbonate phase was absent in the sample at 973K with no cobalt. This suggests that the presence of cobalt is another crucial factor for the recarbonation process to take place. Cobalt ions appear to play a catalytic role. An attempt is made to provide a reasonable explanation of this intriguing recarbonation chemistry, trying to shed some more light on the rather complex chemistry behind the attack of moisture and CO2 to oxide solid surfaces. These results, as a whole, may provide new insight on phenomena observed in the case of some Mn-based catalysts, as the severe deactivation by traces of moisture occurring in hopcalite during CO oxidation at RT. They may also give an additional perspective to environmental processes that involves the interaction of moisture and CO2 with the surface of Mn-based solids or other materials as environmental interfaces.